Technically, this refers to the time prior to the Original Sin – Adam and Eve’s eating of the forbidden fruit. That was the “lapse,” and therefore we speak of the “pre-lapse.”
Colloquially, this means “before things got bad.” Something that is “prelapsarian” represents youthful bliss, ignorant of a darker future. Or perhaps, the moment before disaster strikes.
Merriam-Webster provides some examples:
The vision, which now seems distinctly prelapsarian, was of the Web as a bottom-up phenomenon, with no bosses, and no rewards other than the satisfaction of participating in successful innovation.